Powder coating compositions are a well known and established art. A variety of articles are routinely powder coated by dipping or tumbling them in a suspended or static bed of resinous powder coating or by spray or sprinkling. The article on which the coating is applied is then heated to form a cured powder coating. These coatings have been shown to be particularly useful on metal surfaces including steel, aluminum and iron.
Various powder coating compositions are available that have been used in heat-resistant applications. For example, it is known in the art that aromatic epoxy resins are used to provide good heat resistance. Epoxy compositions or epoxy and silicone mixtures have also been used in the industry for the same purpose. An example of a silicone coating composition is shown in U.S. Pat. No. 5,422,396 (Daly et al.). However, this composition, along with the other silicone based powder coating compositions, rely on the condensation of the silicone resin with another resin or itself to form the coating composition.
Additional references which describe the preparation of heat-resistant protective coatings are described below:
U.S. Pat. No. 4,877,837 (Reising et al.) discloses powder-based compositions comprising glycidyl functional acrylic polymers admixed with silicone having hydroxyl functional groups. The compositions may contain additives such as flow agents, outgas agents and filler pigments (i.e. hydrated silicates of aluminum and calcium metasilicate) and are cured at a temperature between 180.degree. C.-200.degree. C.). PA1 U.S. Pat. No. 4,446,259 (Vasta) discloses coating compositions in a liquid carrier, which can be cured at ambient temperatures, comprising glycidyl functional acrylic polymers admixed with a reactive polysiloxane. The coatings may contain additives such as metal oxides, metallic pigments and trace amounts of multivalent metal ions including tin and zinc. Vasta also discloses fluorocarbon-based powder compositions which are baked at 200.degree. C.-345.degree. C. for about 2 hours to provide a coating 25-75 microns thick. PA1 Other related patents include: U.S. Pat. No. 4,404,042 (Okada et al.); U.S. Pat. Nos. 4,657,963 and 4,746,568 (Matsumoto et al.); U.S. Pat. No. 4,879,344 (Woo et al); and U.S. Pat. No. 5,087,286 (Fukuda et al.). PA1 1) at least one polysiloxane, preferably a hydroxyl-functional siloxane; and PA1 2) at least one adhesion promoter. PA1 1) at least one acrylic polymer; PA1 2) at least one polysiloxane; and PA1 3) at least one inorganic compound,
A drawback with the prior art references relates to the protective coatings appearance and protective nature when elevated temperatures are used to cure high-film-build powder-based compositions. For example, when said compositions are cured at elevated temperatures, condensation and thermal decomposition by-products often generate gases which can form pinholes, cracks and craters in the protective coating. These defects can be detrimental to coating properties including aesthetics, heat resistance and corrosion resistance and typically limit the coating thickness to less than about 100 microns.
In attempting to overcome this disadvantage, users of these silicone based powder coating compositions have to allow for long cure times at low temperatures or perform a controlled cure that occurs by slowly increasing the oven temperature. Some powder coatings comprising silicone resins and its blends also have poor storage stability, i.e., the powder coating composition clumps up, cohesively bonds or sinters within 48 hours of storage.
Another problem which has existed in the application of thermally stable silicone compositions to substrates is that the coating compositions (a coating composition which has been powder coated, with the coating melted and cured to a continuous film at temperatures up to 230.degree. C.) have exhibited poor initial adhesion to the substrates. Even though the adhesion of undamaged coatings to the substrates tends to improve with elevated temperatures during use (e.g., temperatures above 280.degree. C.), damage can occur before such use, and additional curing of the initially coated and cured material is time consuming and expensive for the manufacturer. For example, when thermoset silicone powder coated materials have been dropped or scratched, the coatings may readily peel from the substrate or flake off the substrate. It is desirable to provide a powder coating composition which, in addition to providing good thermal resistance, also provides good initial bond strength to substrates.
There has therefore been a need for a powder coating that provides high heat resistance, does not significantly pit, can be applied with a generally high film build without significantly affecting the coating, and that has good initial bond strength to substrates.